1. Field of the Invention
The present invention relates to a ferromagnetic tunnel magnetoresistive element and a magnetic head using the same.
2. Description of Related Prior Art
Conventionally, ferromagnetic tunnel magnetoresistive (TMR) elements have been proposed as one type of magnetoresistive elements. JP-A-10-4227 describes a magnetic head using a TMR element. However, the magnetoresistance of such a conventional TMR element depends greatly on an applied voltage where a TMR ratio becomes lower as a voltage is applied. In order to employ the TMR element in a magnetic head or a magnetic memory, it is necessary to increase the output while decreasing the dependency thereof on the applied voltage. An increase in the output can be achieved by applying a half-metallic ferromagnet whose degree of spin-polarization is higher than that of a magnetic ferromagnet used in the conventional TMR element. An attempt to increase a TMR ratio in a TMR element using a material associated with a high degree of spin-polarization (La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3) is described in Europhysics Letters, 39(5), pp. 545-549 (1997). Physical Review Letters describes that 40% to 50% TMR ratio can be obtained up to an applied voltage of about 1 V in Co/SrTiO3/La0.7Sr0.3MnO3.
There has been no solution to the decrease in the TMR ratio as well as its dependency on an applied bias voltage. Thus, the present invention has an objective of providing a ferromagnetic tunnel magnetoresistive element whose output is higher and whose magnetoresistive ratio is less dependent on an applied bias voltage compared to conventional elements. In addition, the present invention has an objective of providing a magnetoresistive magnetic head and a magnetic memory device using such a ferromagnetic tunnel magnetoresistive element.
The degree of spin-polarization (P) of electrons is generally understood as a difference between the numbers (densities of states) of electrons in different rotation directions (about their own axes) (where a clockwise spin is referred to as a downward spin and an anticlockwise spin as an upward spin). For example, the degree of spin-polarization P=0.8 indicates that the number of upward spins is nine times higher than that of downward spins.
A half-metallic ferromagnet is a completely polarized ferromagnet having a gap of densities of states of upward and downward 3d electronic spins of about 1 eV. Fermi energy (EF) is present across either one of the densities of state. Since electrons in charge of electronic transport exist at around the Fermi energy, only one of the spins will have a transport property. Thus, the degree of spin-polarization (P) in a half-metallic ferromagnet is 1. On the other hand, ferromagnetic metals such as Co (Co-based alloy), Fe (Fe-based alloy) and Ni (Ni-based alloy) have a degree of polarization of about 0.4, with no gap in the 3d band, and with both upward and downward spins present at Fermi energy.
The magnetoresistance (TMR ratio) of a TMR element may be represented as 2P1P2/(1xe2x88x92P1P2) using the above-mentioned degree of spin-polarization P, where P1 and P2 are degrees of spin-polarization of two respective ferromagnetic layers sandwiching an insulating barrier layer of the TMR element. In order to obtain a high TMR ratio, a half-metallic ferromagnet with a high degree of spin-polarization P (Fe3O4, CrO2, etc.) is advantageously used.
The dependency of the TMR element on an applied bias voltage is known to depend on profiles of the densities of states at interfaces of the two ferromagnetic layers with the insulating barrier layer, within the barrier height. Accordingly, a desirable bias voltage dependency of a TMR ratio can be obtained by appropriately combining the insulating barrier layer with the ferromagnetic layers of the TMR element.
Basically, the present invention has a three-terminal structure including upper ferromagnetic layer/insulating barrier layer/intermediate ferromagnetic layer/insulating barrier layer/lower ferromagnetic layer, each ferromagnetic layer having an electrode terminal. Two electric closed-circuits (for example, a closed-circuit between the upper ferromagnetic layer and the lower ferromagnetic layer, and a closed-circuit between the intermediate ferromagnetic layer and the lower ferromagnetic layer) are provided to vary the bias voltage applied to the tunnel element in one of the closed-circuits, thereby decreasing the bias voltage dependency of a magnetoresistive ratio in the other closed-circuit.
Specifically, the objective of the present invention can be achieved with the following ferromagnetic tunnel magnetoresistive elements.
(1) A ferromagnetic tunnel magnetoresistive element, comprising: a first ferromagnetic layer; a first insulating barrier layer formed on the first ferromagnetic layer; a second ferromagnetic layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the second ferromagnetic layer; and a third ferromagnetic layer formed on the second insulating barrier layer, wherein the element further comprises a terminal for applying a first bias voltage between the first ferromagnetic layer and the third ferromagnetic layer, and a terminal for applying a second bias voltage between the second ferromagnetic layer and the first or third ferromagnetic layer.
(2) A ferromagnetic tunnel magnetoresistive element according to (1), further comprising a first antiferromagnetic layer under the first ferromagnetic layer for fixing the magnetization direction of the first ferromagnetic layer, and a second antiferromagnetic layer on the third ferromagnetic layer for fixing the magnetization direction of the third ferromagnetic layer.
(3) A ferromagnetic tunnel magnetoresistive element according to either one of (1) and (2), wherein the second ferromagnetic layer is formed of a lamination of three ferromagnetic metal layers.
(4) A ferromagnetic tunnel magnetoresistive element according to (1), wherein each of the first and second ferromagnetic layers is formed of a lamination of two ferromagnetic metal layers.
(5) A ferromagnetic tunnel magnetoresistive element according to any one of (1) to (4), wherein at least one of the first, second and third ferromagnetic layers makes contact with a non-magnetic metal layer.
The objective of the present invention can also be achieved with the following magnetic head.
(6) A magnetic head provided with a magnetoresistive element comprising: a first ferromagnetic layer; a first insulating barrier layer formed on the first ferromagnetic layer; a second ferromagnetic layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the second ferromagnetic layer; and a third ferromagnetic layer formed on the second insulating barrier layer, wherein the element further comprises a terminal for applying a first bias voltage between the first ferromagnetic layer and the third ferromagnetic layer, and a terminal for applying a second bias voltage between the second ferromagnetic layer and the first or third ferromagnetic layer.
(7) A magnetic head according to (6), wherein the element further comprises a first antiferromagnetic layer under the first ferromagnetic layer for fixing the magnetization direction of the first ferromagnetic layer, and a second antiferromagnetic layer on the third ferromagnetic layer for fixing the magnetization direction of the third ferromagnetic layer.
(8) A magnetic head according to either one of (6) and (7), wherein the second ferromagnetic layer is formed of a lamination of three ferromagnetic metal layers.
(9) A magnetic head according to (6), wherein each of the first and second ferromagnetic layers is formed of a lamination of two ferromagnetic metal layers.
(10) A magnetic head according to any one of (6) to (9), wherein at least one of the first, second and third ferromagnetic layers makes contact with a non-magnetic metal layer.
Furthermore, the objective of the present invention can be achieved with the following ferromagnetic tunnel magnetoresistive element.
(11) A ferromagnetic tunnel magnetoresistive element, comprising: a first half-metallic ferromagnetic layer; a first insulating barrier layer formed on the first half-metallic ferromagnetic layer; a ferromagnetic metal layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the ferromagnetic metal layer; and a second half-metallic ferromagnetic layer formed on the second insulating barrier layer, wherein the element further comprises a terminal for applying a first bias voltage between the first half-metallic ferromagnetic layer and the second half-metallic ferromagnetic layer, and a terminal for applying a second bias voltage between the ferromagnetic metal layer and the first or second half-metallic ferromagnetic layer.
(12) A ferromagnetic tunnel magnetoresistive element according to (11), further comprising a first antiferromagnetic layer under the first half-metallic ferromagnetic layer for fixing the magnetization direction of the first half-metallic ferromagnetic layer, and a second antiferromagnetic layer on the second half-metallic ferromagnetic layer for fixing the magnetization direction of the second half-metallic ferromagnetic layer.
(13) A ferromagnetic tunnel magnetoresistive element according to either one of (11) and (12), wherein the ferromagnetic metal layer is formed of a lamination of three ferromagnetic metal layers.
(14) A ferromagnetic tunnel magnetoresistive element, comprising: a first ferromagnetic metal layer; a first insulating barrier layer formed on the first ferromagnetic metal layer; a half-metallic ferromagnetic layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the half-metallic ferromagnetic layer; and a second ferromagnetic metal layer formed on the second insulating barrier layer, wherein the element further comprises a terminal for applying a first bias voltage between the first ferromagnetic metal layer and the second ferromagnetic metal layer, and a terminal for applying a second bias voltage between the half-metallic ferromagnetic layer and the first or second ferromagnetic metal layer.
(15) A ferromagnetic tunnel magnetoresistive element according to (14), wherein each of the first and second ferromagnetic metal layers has a lamination of two ferromagnetic metal layers.
(16) A ferromagnetic tunnel magnetoresistive element according to (14), further comprising a first antiferromagnetic layer under the first ferromagnetic metal layer for fixing the magnetization direction of the first ferromagnetic metal layer, and a second antiferromagnetic layer on the second ferromagnetic metal layer for fixing the magnetization direction of the second ferromagnetic metal layer.
(17) A ferromagnetic tunnel magnetoresistive element according to any one of (11) to (16), wherein the half-metallic ferromagnetic layer is an oxide or a compound comprising Fe, Co or Mn.
(18) A ferromagnetic tunnel magnetoresistive element, comprising: a first antiferromagnetic layer; a half-metallic ferromagnetic layer formed on the first antiferromagnetic layer; a first insulating barrier layer formed on the first half-metallic ferromagnetic layer; a first ferromagnetic metal layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the first ferromagnetic metal layer; a second ferromagnetic metal layer formed on the second insulating barrier layer; and a second antiferromagnetic layer formed on the second ferromagnetic metal layer, wherein the element further comprises a terminal for applying a first bias voltage between the half-metallic ferromagnetic layer and the second ferromagnetic metal layer, and a terminal for applying a second bias voltage between the first ferromagnetic metal layer and the half-metallic ferromagnetic layer or the second ferromagnetic metal layer.
(19) A ferromagnetic tunnel magnetoresistive element according to (18), wherein the first ferromagnetic metal layer is formed of a lamination of three ferromagnetic metal layers.
(20) A ferromagnetic tunnel magnetoresistive element, comprising: a first antiferromagnetic layer; a first ferromagnetic metal layer formed on the first antiferromagnetic layer; a first insulating barrier layer formed on the first ferromagnetic metal layer; a second ferromagnetic metal layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the second ferromagnetic metal layer; a third ferromagnetic metal layer formed on the second insulating barrier layer; and a second antiferromagnetic layer formed on the third ferromagnetic metal layer, wherein the element further comprises a terminal for applying a first bias voltage between the first ferromagnetic metal layer and the third ferromagnetic metal layer, and a terminal for applying a second bias voltage between the second ferromagnetic metal layer and the first or third ferromagnetic metal layer.
(21) A ferromagnetic tunnel magnetoresistive element according to any one of (11) to (20), wherein each of the first and second insulating barrier layers is made of an oxide or a compound comprising at least one of Al, Mg, Ti, Ta, Hf, Nb, Mo, Cr, Ga and As.
The objective of the present invention can also be achieved with the following magnetic head.
(22) A magnetic head provided with a magnetoresistive element comprising: a first half-metallic ferromagnetic layer; a first insulating barrier layer formed on the first half-metallic ferromagnetic layer; a ferromagnetic metal layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the ferromagnetic metal layer; and a second half-metallic ferromagnetic layer formed on the second insulating barrier layer, wherein the element further comprises a terminal for applying a first bias voltage between the first and second half-metallic ferromagnetic layers, and a terminal for applying a second bias voltage between the ferromagnetic metal layer and the first or second half-metallic ferromagnetic layer.
(23) A magnetic head according to (22), wherein the element further comprises a first antiferromagnetic layer under the first half-metallic ferromagnetic layer for fixing the magnetization direction of the first half-metallic ferromagnetic layer, and a second antiferromagnetic layer on the second half-metallic ferromagnetic layer for fixing the magnetization direction of the second half-metallic ferromagnetic layer.
(24) A magnetic head according to either one of (22) and (23), wherein the ferromagnetic metal layer is formed of a lamination of three ferromagnetic metal layers.
(25) A magnetic head provided with a magnetoresistive element comprising: a first ferromagnetic metal layer; a first insulating barrier layer formed on the first ferromagnetic metal layer; a half-metallic ferromagnetic layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the half-metallic ferromagnetic layer; and a second ferromagnetic layer formed on the second insulating barrier layer, wherein the element further comprises a terminal for applying a first bias voltage between the first ferromagnetic metal layer and the second ferromagnetic metal layer, and a terminal for applying a second bias voltage between the half-metallic ferromagnetic layer and the first or second ferromagnetic metal layer.
(26) A magnetic head according to (25), wherein each of the first and second ferromagnetic metal layers is formed of a lamination of two ferromagnetic metal layers.
(27) A magnetic head according to (25), wherein the element further comprises a first antiferromagnetic layer under the first ferromagnetic metal layer for fixing the magnetization direction of the first ferromagnetic metal layer, and a second antiferromagnetic layer on the second ferromagnetic metal layer for fixing the magnetization direction of the second ferromagnetic metal layer.
(28) A magnetic head according to any one of (22) to (27), wherein the half-metallic ferromagnetic layer is an oxide or a compound comprising Fe, Co or Mn.
(29) A magnetic head provided with a magnetoresistive element comprising: a first antiferromagnetic layer; a half-metallic ferromagnetic layer formed on the first antiferromagnetic layer; a first insulating barrier layer formed on the first half-metallic ferromagnetic layer; a first ferromagnetic metal layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the first ferromagnetic metal layer; a second ferromagnetic metal layer formed on the second insulating barrier layer; and a second antiferromagnetic layer formed on the second ferromagnetic metal layer, wherein the element further comprises a terminal for applying a first bias voltage between the half-metallic ferromagnetic layer and the second ferromagnetic metal layer, and a terminal for applying a second bias voltage between the first ferromagnetic metal layer and the half-metallic ferromagnetic layer or the second ferromagnetic metal layer.
(30) A magnetic head according to (29), wherein the first ferromagnetic metal layer is formed of a lamination of three ferromagnetic metal layers.
(31) A magnetic head provided with a magnetoresistive element comprising: a first antiferromagnetic layer; a first ferromagnetic metal layer formed on the first antiferromagnetic layer; a first insulating barrier layer formed on the first ferromagnetic metal layer; a second ferromagnetic metal layer formed on the first insulating barrier layer; a second insulating barrier layer formed on the second ferromagnetic metal layer; a third ferromagnetic metal layer formed on the second insulating barrier layer; and a second antiferromagnetic layer formed on the third ferromagnetic metal layer, wherein the element further comprises a terminal for applying a first bias voltage between the first ferromagnetic metal layer and the third ferromagnetic metal layer, and a terminal for applying a second bias voltage between the second ferromagnetic metal layer and the first or third ferromagnetic metal layer.
(32) A magnetic head according to any one of (22) to (31), wherein each of the first and second insulating barrier layers is made of an oxide or a compound comprising at least one of Al, Mg, Ti, Ta, Hf, Nb, Mo, Cr, Ga and As.
In using the ferromagnetic tunnel magnetoresistive element, a current is provided in a thickness direction of the layers.
Moreover, the ferromagnetic tunnel magnetoresistive element is preferably formed on an alignment film. Examples of the alignment film include an oxide or a compound containing at least one of Ni, Zr, Zn, Al, Mg, Ti, Ta, Hf, Nb, Mo, Cr and Co.